1. Field of the Invention
The present invention relates to audio frequency information encoding and decoding, and more particularly relates to audio message encoding and recording on a recording medium for later retrieval, decoding and playback with related video information which is recorded with the audio information on the same recording medium.
2. Brief Description of the Prior Art
"Stop-motion" is a technique in the playback of recorded video information whereby a single frame of recorded video signals is played repeatedly to provide a continuous video picture of the visual information contained in the single frame being played. Stop-motion techniques are known and are in widespread use in TV broadcasting, the most well-known example being in the area of televised sports broadcasts. In such broadcasting applications, the recording medium generally used to create the stop-motion effect is the video tape.
A development which has made the stop-motion capability attractive for applications other than broadcasting is the optical disc. An optical disc is a flat disc approximately the size of an LP phonograph record which is made of clear plastic and is capable of having information recorded on an imbedded surface in the interior of the disc in form of spiralling or circular tracks of optically readable indicia. Optical discs are read by imaging a beam of light to a tiny spot on a track, rotating the disc and thus causing the spot of light to scan linearly along the track, and detecting with a photodetector the amount of light which emerges from the track in a selected direction. Information is stored in the disc in the pattern in which the indicia are provided on the track. As the disc is scanned by the spot of light, the amount of light which is detected varies in accordance with the alternate presence and absence of the indicia, and the information is recovered by detecting the electrical signal variations of the photodetector output produced by the particular pattern of indicia on the track.
The most widely-used format for the recording and playback of video information on an optical disc involves frequency modulating a video carrier and one or more audio subcarriers, combining the frequency-modulated carrier and subcarriers, and then varying the spatial frequency and relative length of the indicia, as compared with the areas between them, in accordance with the frequency-modulated carrier and subcarrier signals.
It is possible to record video information on an optical disc such that areas of the track corresponding to vertical sync intervals are aligned radially on the disc. Such discs are called Constant Angular Velocity ("CAV") discs because in the recording and playback of video information, such discs are rotated at a constant angular velocity.
CAV discs provide several useful features which result from the fact that the vertical sync intervals in every track on the disc line up in the same radial direction. This arrangement makes it possible while reading the disc to jump from track to track with relative ease while maintaining the synchronization of the horizontal and vertical sync oscillator circuits of the television or monitor being driven by the disc player output. This is made possible because, as the spot of light arrives at a new track after being jumped from a previous track, the synchronization of the video information recorded in that track is identical to the synchronization of the video information in the track from which the spot was just removed. Thus, after making such a jump there is no need to reestablish lost synchronization, and instead the playing of the video information can proceed smoothly and without interruption.
This capacity to jump smoothly among frames of video information has made the optical disc a highly suitable recording medium for video information intended for play in the stop-motion mode. For example, an entire optical disc can be recorded exclusively with stop-motion video information. In such a case each video frame recorded on the disc contains a different picture, and the disc can be read frame by frame, and thus picture by picture, as one would read a book, by playing each frame in stop-motion mode and accesing individual frames as desired. Considering that over 50,000 frames of video can be stored on a single side of a CAV disc, the utility of such a format is obviously very wide. For example, an entire department store catalog or an entire 100,000 picture educational program can be placed on a single optical disc.
Stop-motion features of the optical disc become even more attractive when combined with the capability of audio playback during the stop-motion playing of the video information.
Techniques have been devised for the recording of audio information for playback with a frame of stopmotion video. According to one technique, set forth in co-pending U.S. patent application Ser. No. 202,840, filed on Oct. 31, 1980, issued on July 31, 1984 as U.S. Pat. No. 4,463,389 in the name of Scott M. Golding, incorporated herin by reference, and commonly assigned to the assignee of the present invention "stop-motion audio" to be played back with an accompaying stop-motion frame of video is encoded in a digitized form, for example by adaptive delta modulation, and recorded on one of two available audio channels on the disc. During playback, prior to the playback of the associated stop-motion video frame, the digitally encoded stop-motion audio information is read from the audio channel and stored in a storage device such as a RAM. When the stop-motion frame is played, the digitized audio information is read out of the storage device, decoded and played along the stop-motion video.
One of the limitations of this technique is that the bit rate of the digitized audio data as it is read from the disc must be kept within the bandwidth limitations of the audio channel on which it is recorded. A typical value which has been used with this technique is a read bit rate of twelve kilohertz. When employing adaptive delta modulation, sampling bit rates in the encoding process are typically sixteen kilohertz or greater to provide desired intelligibility. Thus, according to this technique, the disc must be played in the normal mode of operation for a period of time just slightly longer than the duration of the stop-motion message in order to read the encoded stop-motion message in memory. The technique is therefore not useful for recording programs having a large number of closely-spaced stop-motion frames. It does, however, provide a relatively low-cost way of recording and playing back stop-motion audio information where stop-motion frames are provided at more widely-spaced intervals throughout the program.
A second technique, described in co-pending U.S. patent application Ser. No. 066,620, filed on Aug. 15, 1979 now abandoned in favor of Ser. No. 161,231, filed June 18, 1980, in the name of Wayne R. Dakin one of the inventors of the present invention and commonly assigned to the assignee of the present invention, also involves the encoding of stop-motion audio information, for example by adaptive delta modulation. However, the digitally encoded stop-motion audio information is recorded in the place of video information on one or more successive frames. The stop-motion audio information message is encoded at a desired sampling rate, such as sixteen kilohertz and then time compressed to a bit rate of 7.2 megahertz and encoded such that the bandwidth is within the capabilities of the video electronic circuitry. The encoded data is then provided in the place of the video information in the horizontal lines of the video frames. The large time compression of the digitized audio information permits the storage of stop-motion audio messages of a duration of up to eleven seconds in the video data portion of a single frame of video. The electronic circuitry required to implement this second technique is more costly than that associated with the first technique described above, but the later techniques permits considerably more stop-motion audio message information to be stored on an optical disc. Thus, an optical disc can be formatted in an alternating sequence of stop-motion video frames and stop-motion audio frames to provide each video frame with a stop-motion audio message of up to eleven seconds in duration. This "video encoding" technique, therefore, represents an enormous improvement in the efficiency of storing digitally encoded stop-motion audio messages on an optical disc.
The video encoding technique has certain limitations, however. Program demands for stop-motion message duration vary considerably. Video programmers frequently require only two or three seconds for a particular stop-motion frame, but occasionally require a stop motion audio message of up to twenty seconds or more. This presents problems in selecting a format for the stop-motion audio information. For reasons of economy it is desirable to have a standard format for stop-motion audio information recording and playback so that video optical disc players having stop-motion capability do not have to be redesigned for every new program. A reasonable compromise standard format for stop-motion audio encoding is two successive frames of video field devoted to a single stop-motion audio message sampled at a rate of sixteen kilohertz. Such a format enables the storage and playback of stop-motion audio messages of up to twenty-two seconds in duration at a reasonable level of intelligibility. This permits the storage and playback of all but the most lengthy stop-motion audio messages in most program applications. However, most stop-motion audio messages are considerably less in length, some lasting for only two or three seconds, as was mentioned previously. For such stop-motion messages an enormous amount of storage capability is wasted. Even if only a single frame were to be devoted to a single stop-motion audio message, the eleven seconds afforded at a sixteen kilohertz bit rate would still be excessive for many stop-motion audio messages, and longer messages could not be recorded in such a format.
In addition, while a sixteen kilohertz bit rate in connection with adaptive delta modulation is one which provides a reasonable compromise between intelligibility and data packing density requirements, audio message information played back at a sixteen kilohertz sampling rate does not provide full fidelity. Frequently it is desirable to provide such increased intelligibility, but prior art techniques do not provide the flexibility to do so.
Accordingly, it will be appreciated that there is a need for a video recording and playback system having stop-motion audio recording and playback capabilities which overcome the above-noted limitations. In particular, there is a need for such a system which provides more flexibility in the provision of stop-motion audio messages on a recording medium while preserving standardization to permit the economical manufacture of the associated apparatus.
The present invention fulfills these needs.